1. Field of the Invention
The present invention relates to a power supply circuit, amplification circuit and composite (hybrid) integrated circuit system, and more particularly to improvements in such circuits for increasing the efficiency of an audio amplifier.
2. Description of the Prior Art
Audio amplifiers according to the prior art will now be described with reference to FIGS. 1 through 8. In prior art audio amplifiers, increasing efficiency has been an outstanding problem.
The audio amplifiers according to the prior art generally used a circuit such as that shown in FIG. 1. This circuit includes a push-pull circuit defined by the final stage transistors Q1 and Q2. An audio signal from the preceding stage (which is normally a preamplifier) is inputted into the transistors Q1 and Q2 at their base. The transistors Q1 and Q2 amplify such an audio signal to drive a loudspeaker SP through power from positive and negative power sources (+Vcc, -Vcc).
In such an arrangement, the final stage transistors Q1 and Q2 are driven respectively by the positive and negative supply voltages (+Vcc, -Vcc), the magnitudes of which can fetch the maximum output at all times. If the level of the audio signal inputted thereinto is relatively low, therefore, it has a disadvantage in that the power loss may be increased in the final stage transistors Q1 and Q2.
In order to avoid such a problem, there has been proposed a PWM (Pulse Width Modulation) amplifier such as is shown in FIG. 2. Such a PWM amplifier is adapted to input an audio signal from a preamplifier into a PWM circuit 1, to prepare a PWM signal having a duty depending on the signal level in the waveform of the audio signal at each point, and to drive MOS type transistors Q10 and Q20 defining an output stage CMOS inverter for switching supply power from positive and negative power sources (+Vcc, -Vcc) to a loudspeaker SP.
Such an arrangement can improve the efficiency of the system since the MOS type transistors Q10 and Q20 are switchingly driven to prolong the off-time period in the transistors Q10 and Q20 when the level of the input audio signal is relatively low. However, the amplifying circuit of FIG. 2 must include a low-pass filter circuit 2 inserted upstream of the loudspeaker SP to demodulate the PWM signal from the transistors Q1 and Q2 into an audio signal; the low-pass filter comprising a coil L and a capacitor C. Therefore, the PWM amplifier cannot follow the sharp rising to the larger amplitude of the audio input signal since the rise speed (slew rate) in the amplifier is too slow. Since the carrier components in the PWM signal are not completely removed and will be radiated from the output of the amplifying circuit toward the circumference through the signal line connected to the loudspeaker SP, there occurs a problem in that the radiated carrier components adversely affect peripheral instruments, by, for instance, wave disturbance.
For the same purpose, there has also been proposed a circuit such as is shown in FIG. 3. Such a circuit is adapted to amplify an audio signal through a power amplifier 8 comprising a preamplifier 3 and transistors Q1 and Q2 which define a final stage push-pull circuit. The supply voltages (+Vc, -Vc) of the transistors Q1 and Q2 vary depending on the state of the amplified signal.
On operation, an offset voltage is formed by superimposing a constant voltage from an offset power source 4 over the signal PS amplified by the preamplifier 3 and the power amplifier 8. The offset voltage is then inputted into one input of a comparator 6. At the same time, the other input of the comparator 6 receives a supply voltage (+Vc) which is the output voltage of a chopper power source 7.
The comparator 6 compares the offset voltage with the supply voltage (+Vc); the resulting output thereof is used to energize the chopper power source 7 so that the supply voltage (+Vc) will follow the offset voltage.
To actually realize such an operation, a circuit such as is shown in FIG. 4 can be used. The circuit includes a constant voltage generating circuit 9 for maintaining a constant base potential at a transistor Q3, which is a source of constant current supply. Thus, a constant collector current (Ic) flows from the transistor Q3 so that the comparator 6 will also be supplied with a constant current from a transistor Q4.
As the amplified signal PS, which is the output of the power amplifier, is inputted into the base of a transistor Q5 which defines the input of the offset power source 4, the emitter potential in the transistor Q5 varies depending on the magnitude of the amplified signal PS. As a result, the potential at a point A in FIG. 4 becomes equal to a potential difference at a resistor R0 or a product of the collector current Ic of the transistor Q3 and the resistance R0 plus the emitter potential of the transistor Q5.
Therefore, the potential at the point A in FIG. 4 becomes equal to an offset voltage obtained by adding the constant potential difference at the resistor R0 to the amplified signal PS. This offset voltage will then be inputted into the input of the comparator and compared with the supply voltage (+Vc) which is the output of the chopper power source.
If the supply voltage (+Vc) is lower than the offset voltage, the output of the comparator 6 becomes a high level so that a switching element SW in the chopper power source will be turned on to increase the supply voltage (+Vc). If the supply voltage (+Vc) is higher than the offset voltage, the output of the comparator 6 becomes a low level so that the switching element SW in the chopper power source will be turned off to decrease the supply voltage (+Vc). Therefore, the supply voltage (+Vc) will follow the offset voltage over which a constant voltage is superimposed, as shown in FIG. 5.
In such a manner, the supply voltages (+Vc, -Vc) can be reduced when the input level is lower while the supply voltages (+Vc, -Vc) can be increased when the input level is higher. Consequently, the power loss in the final stage transistors Q1 and Q2 can be controlled to improve the efficiency when the input level is lower.
However, the audio amplifiers shown in FIGS. 3 and 4 raise the following problems.
As an amplified signal ZS having its magnitude substantially equal to the positive power supply (+Vcc) for driving the power supply unit is inputted into the base of the transistor Q5 in such a circuit arrangement as shown in FIG. 4, the potential at the point A in FIG. 4 reaches the base potential in the transistor Q3 through addition of the potential difference across the resistor R0 to this larger amplified signal ZS. Thus, the transistor Q3 will be saturated to clip the offset voltage with the base potential in the transistor Q3.
As a result, the transistors Q3 and Q4 respectively supplying the constant currents to the offset power source 4 and comparator 6 will not operate in the normal manner. The offset power source 4 and comparator 6 become inoperative. Thus, the power amplifier cannot accommodate itself to an amplified signal ZS which is substantially equal to the positive supply power (+Vcc). This raises a problem in that the dynamic range of the amplifier cannot be sufficiently secured.
Furthermore, if the amplified signal rises sharply in such a circuit as shown in FIG. 3, the power supply to the transistors Q1 and Q2 will not be able to follow the sharp rising of the amplified signal.
It is further shown in FIG. 6 that the output of the chopper power source follows the offset voltage generated by half-wave clipping the amplified signal. However, the output of the chopper power source requires an abrupt change of voltage when the amplified signal varies across the ground level. Such a change of voltage becomes more abrupt as the amplitude of the amplified signal becomes larger and as the frequency becomes higher. However, the output of the chopper power source includes a filter circuit comprising a choke coil and a capacitor, which determines the limit of the follow-up to the offset voltage. With the abrupt change of voltage, the output of the chopper power source cannot follow the offset voltage so the output of the amplifier may be clipped within a time zone (HT) in which there is a distortion as shown in FIG. 6.
Further, one of the transistors defining the output stage of the amplifier which has a polarity opposite to that of the directed signal (for example, a transistor Q1 of FIG. 3 toward the negative side of which the amplified signal is directed) does not substantially supply current to a load. At this time, the offset voltage becomes a very low constant voltage, as shown in FIG. 6. If the audio signal is of high-frequency, however, the energy accumulated in the choke coil and capacitor will not be fully consumed as the amplified signal reaches its peak. As shown in FIG. 7, therefore, the output of the chopper power source will include a residual voltage even if the amplified signal is directed to the opposite side. Since such a residual voltage depends on the frequency of the amplified signal, the phase relative to the aforementioned HT signal will vary depending on the frequency of the amplified signal.
In such a manner, the circuit of the prior art raises a further problem in that as the amplified signal abruptly varies, the voltage of the chopper power source cannot follow it. This results in clipping of the amplified signal with a phase depending on the frequency of the amplified signal.